Oil-field waters produced from Middle to Lower Pennsylvanian and Mississippian stratigraphic units in several localities in Ness, Hodgeman, Ford, and Clark Counties of Kansas were analyzed for their strontium isotopic compositions to determine the influence of associated rocks on the chemical characters of the fluids. The ⁸⁷Sr/⁸⁶Sr ratios of these oil waters, ranging between 0.710 and 0.727, are significantly higher than the

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⁸⁷Sr/⁸⁶Sr ratios of any seawater during the Phanerozoic. The lithologic character of the host reservoir rocks had little influence on the strontium isotopic composition of the oil waters. The ⁸⁷Sr/⁸⁶Sr ratios of the waters, at least those from Ness, Hodgeman, and Ford Counties, correlate positively with Sr/Ca ratios (2 × 10^-2 to 10 × 10^-2) and negatively with total dissolved solid (TDS) contents (32 to 48 g/L). The results of the strontium isotopic investigation have several implications for understanding migration and accumulation histories of the oils. The strontium isotopic data can be useful in providing information on the hydrologic relationship among oil pools in an oil field and in determining the genet c relationship among oils produced from stratigraphically separate reservoirs within an oil field. The strontium isotopic data of oil-field waters are useful potentially in differentiating an oil-bearing reservoir from an oil-barren reservoir within an oil field.

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